The broad objectives of the proposed project are: 1) to study quantitatively the dose of irradiation which is needed to control or sterilize metastases at different time intervals after the onset of the metastatic foci, 2) to establish the relationship between the tumor lethal dose for clinically detectable primary tumor and the dose sufficient to control the metastatic foci at both subclinical (microscopic) and clinical (macroscopic) levels, 3) to study the effect of the method of treatment of the primary tumor (surgery and/or radiation) on the frequency and radiation response of the metastatic foci, and 4) to investigate the use of a 5 percent carbon dioxide: 95 percent oxygen mixture, a known radiosensitizer, on the radiation response of both microscopic and macroscopic metastases. For these studies, a mouse model for pulmonary metastases will be employed. Basically, tumor cell suspensions of C3HBA mammary adenocarcinoma injected intravenously result in the formation of pulmonary colonies or metastases. The time interval between injection and treatment will determine the size of the metastatic foci at the time of treatment, that is, whether the lesions are microscopic or macroscopic.